Solar cells attract attention as a powerful environmentally friendly energy source as the energy issue is currently increasingly serious. Currently, inorganic substances such as single crystal silicon, polycrystal silicon, amorphous silicon and compound semiconductors are used as semiconductor materials for photovoltaic elements in solar cells. However, solar cells produced using an inorganic semiconductor have not become widespread in ordinary households yet because they have a higher cost compared to power generation systems such as thermal power generation. The factor of the high cost is mainly the process of forming a semiconductor thin-film under vacuum and a high temperature. Thus, organic solar cells produced using an organic semiconductor such as a conjugated polymer or an organic crystal, or an organic pigment, as a semiconductor material expected to ensure simplification of the production process are being studied. In those organic solar cells, a semiconductor material can be prepared by a coating method and, therefore, the production process can be considerably simplified.
However, organic solar cells produced using a conjugated polymer or the like have not been put into practical use yet because they have lower photoelectric conversion efficiency and durability compared to conventional solar cells produced using an inorganic semiconductor. To put organic solar cells into practical use, it is absolutely necessary to increase durability so that high photoelectric conversion efficiency is maintained for a long period of time.
As one method of improving photoelectric conversion efficiency and durability of an organic solar cell, an electron extraction layer is placed between a power generation layer and a cathode. As an electron extraction layer, for example, one including titanium oxide (Japanese Patent No. 05298308) or zinc oxide (National Publication of International Patent Application No. 2013-55125) has been reported.
In addition, it is disclosed that by applying an ethanolamine solution onto a zinc oxide layer that is an electron extraction layer, the surface energy level is adjusted to improve photoelectric conversion efficiency (“Advanced Materials”, 2014, Vol. 26, pages 494-500).
We believed that an electron extraction layer including an inorganic oxide in combination with an electron-donating compound, typically an amine-based material, is effective in improving photoelectric conversion efficiency. However, in an element configuration using an electron extraction layer including zinc oxide in combination with an ethanolamine solution as disclosed in “Advanced Materials”, 2014, Vol. 26, pages 494-500, the surface energy level on zinc oxide is adjusted to stabilize the interface state and, therefore, an element is driven with high photoelectric conversion efficiency immediately after preparation of the element, but photoelectric conversion efficiency is gradually reduced due to accumulation of thermal loads. In other words, the photovoltaic element described in “Advanced Materials”, 2014, Vol. 26, pages 494-500 does not have sufficient heat stability although the photovoltaic element exhibits higher heat stability compared to a photovoltaic element produced using zinc oxide alone as an electron extraction layer.
It could therefore be helpful to provide a photovoltaic element having high photoelectric conversion efficiency, and excellent heat stability and durability.